It has been reported that cholesteatoma is more aggressive in children than in adults and that it affects boys more often than girls. We found particularly prominent sex differences in the clinical characteristics and behavior of cholesteatoma in children, by comparing them with those of acute otitis media (AOM), otitis media with effusion (OME) and adult cholesteatoma. We retrospectively analyzed the clinical records of children diagnosed as having cholesteatoma by comparing their courses with those of AOM, OME, chronic otitis media, microtia and congenital malformation of the ossicles. The boy-dominant tendency were observed in OMA, OME, pediatric cholesteatoma, microtia and congenital malformation of the ossicles. Cholesteatoma has a tendency to fill the middle ear cleft in boys, in whom the most extensive cholesteatomas were observed. If OMA, OME, and cholesteatoma in children are considered to be a series of inflammatory middle ear diseases, a common factor must be involved in the boy-dominant tendency for this series of inflammatory middle ear diseases. We have proposed six factors contributing to sex differences in pediatric cholesteatoma. Among these, immunological, environmental and congenital factors were thought to be responsible for the sex differences.